The present invention relates to networked systems composed of a plurality of devices clustered for the exchange of data and control messages formatted according to predetermined protocols and, in particular, to such systems where inter-device communication is via wireless link. The invention further relates to devices for use in groups or clusters to form such systems.
Networked interconnection of devices has long been known and used, starting from basic systems where different system functions have been provided by separate units, for example hi-fi systems or security systems having detectors, a control panel and one or more alarm sounders. A development has been the so-called home bus systems where a greater variety of products have been linked with a view to providing enhanced overall functionality for example domestic audio/video apparatus coupled with a home security system and the use of telephone. An example of such a home bus system is the domestic digital bus (D2B), the communications protocols for which have been issued as standard IEC 1030 by the International Electrotechnical Commission in Geneva, Switzerland. The D2B system provides a single wire control bus to which all devices are interfaced with messages carried between the various devices of the system in a standardised form of data packet.
Even with a single wire link from device to device, the amount of inter-connection can become unwieldy leading to installation problems for the user, especially where a cluster of devices to be linked is spread across two or three rooms. In order to get around some of these problems, the use of radio frequency or infra-red communications has been proposed to replace the wired links with a central base station receiving messages from one device and forwarding to their destination device. The need for a base station introduces its own problems, not the least of which is the fact that each message must be transmitted twice, namely from its source to the base station and from the base station to its target trasmitting each message twice this not only increases the volume of traffic but also increases the risk of transmission errors. In terms of consumer systems, the user is constrained to buy the base station first and, being the most complex part of the system, it will generally be the most expensive. There is also the problem common to all such centralized systems, namely that if the base station fails, the whole system becomes unusable.
While infra-red communication has some problems in terms of line-of-sight constraints leading to limitation on the relative positions of the devices to be interconnected, radio frequency communications can pass through walls, doors and ceilings, which makes their use particularly attractive where clusters spread over two or more rooms. This improved range leads to its own problems, such as RF signals spreading into neighbouring properties where there may be compatible systems operating on the same frequency and according to the same set of communications protocols. This can lead to inter-system interference and control problems.
In order to get around the problem of interference to allow users to operate overlapping clusters, it is necessary to provide all devices within range of the RF transmissions with a unique identifier (device address) so that devices can ignore messages carrying identifiers other than their own. Various techniques for unique device address generation have been proposed, including providing sets of switches so that a user can set different codes for each device they own. Such a set up procedure is cumbersome for the user and still leaves a need for the user to consult with their neighbours over address selection in order to avoid duplication. Systems using random number selectors are a great improvement in terms of simplifying the set up procedure for the user. However, these systems have a slight drawback in that they rely on the probability that the number they select will be locally unique. The probability that the selected number will be locally unique will, of course, drop as the number of devices in the locality increases. The probability of unique selection can be increased by lengthening the number of digits in the random number but this has its own penalty in terms of increased system complexity, both in the generation and handling of longer device addresses, and a locally unique number may still not be selected.